1. Field of Invention
The invention relates to a navigation system and also to a program and a storage medium thereof.
2. Description of Related Art
A navigation system searches for an optimum route that gives the lowest sum of search costs assigned to respective link roads along a route to a destination, and provides navigation guidance to the destination along the determined optimum route. In some cases, an accidental or unpredictable obstacle, such as road repairs or congestion, occurs during navigation along a route. Japanese Unexamined Patent Application Publication No. 10-221100, which is incorporated herein by reference in its entirety, discloses a navigation system having an additional function that selects an alternative route when an obstacle such as road repair or congestion occurs.
In the method of determining an alternative route in Japanese Unexamined Patent Application Publication No. 10-221100, the user is allowed to select a distance from the current position of the vehicle while increasing the search cost. The increment of the search cost decreases as the distance between the road section of the original route and the obstacle increases. The increase of the search cost relating to the original road section, caused by the described method, enables the system to produce an alternative route from the current position of the vehicle, i.e., the position of the obstacle on the road to a certain point on the original route.
However, with such conventional navigation systems, many vehicles encountered by an obstacle try to avoid the obstacle by using the same alternative route, and thus congestion also occurs on the alternative route. As a result, it becomes necessary to search for another alternative route. Thus, the operation has to be repeatedly performed.